1. Field of the Invention
The present invention relates to an optical disc apparatus for use to read and/or write data from/on an optical disc using a light beam. More specifically, the present invention relates to an optical disc drive which can read and/or write data with high density by accurately detecting and correcting a spherical aberration caused by a change in the thickness of a protective layer of an optical disc particularly when a light beam is converged using a lens or the like with a large numerical aperture.
2. Description of the Related Art
Optical recording medium have been conventionally proposed as recording mediums for storing information such as video information, audio information, or programs and data for computers. The optical disc drive can read and/or write data from/on the optical recording medium by using optical system. For example, the following mediums are known: read-only/writable type optical discs represented by a CD (Compact Disc), a DVD (Digital Versatile Disc), and a BD (Blu-ray Disc), a PD (phase-change optical Disc), an MO (Magneto Optical) disc, and optical cards.
In the following explanation, an optical recording medium represents an optical disc. For example, as shown in FIG. 1, the optical disc has an information storage layer 29 for storing information. An optical disc drive writes data on the information storage layer 29 and read the data from the information storage layer 29 by using optical means. The information storage layer 29 is protected by a protective layer 25.
Initially, referring to FIG. 14, a configuration of a conventional optical disc drive will be discussed below. Such an optical disc drive is described in, for example, Japanese Patent Laid-Open Publication No. 2002-190125.
FIG. 14 shows the configuration of the functional block of a conventional optical disc drive 140. A disc motor 10 of the optical disc drive 140 rotates an optical disc 20 as an information carrier, with a predetermined number of revolutions. A light beam 30 emitted from a semiconductor laser light source 3 is converged to the information storage layer of the optical disc 20 by an objective lens 1. A focus actuator 2 moves the objective lens 1 in a substantially perpendicular direction (focusing direction) to an information storage layer 29 of the optical disc 20 so as to change the convergence position of the light beam. As a result, a beam spot is formed on a desired position of the information storage layer. The control of a convergence position by using the focus actuator 2 is called “focus control”.
The light beam 30 reflected by the information storage layer 29 of the optical disc 20 passes through the objective lens 1 and is received by a photodetector 4, and the reflected light beam 30 is detected as photocurrent of a level corresponding to a quantity of received light. The objective lens 1 is adjusted in consideration of the influence caused by the thickness of the protective layer of the optical disc 20. To be specific, a correction quantity of spherical aberration is adjusted on the assumption that focus control is stably performed on the information storage layer 29 of the optical disc 20, thereby obtaining a high-quality information signal. The “spherical aberration” means a displacement between the focal position of light passing through the inside of the objective lens 1 and the focal position of light passing through the outside of the objective lens 106.
The following will describe the detail of the focus control and spherical aberration control which relates to the correction of a spherical aberration.
Referring to FIG. 15, the focus control will be firstly described below. Note that the following optical system constitutes a detection system which detects a focus error by a typical astigmatism method. FIG. 15 shows a detailed configuration of the photodetector 4 and a preamplifier 11. The photodetector 4 for detecting photocurrent includes an outer peripheral photodetector 40 and an inner photodetector 41, and each of the receiving parts has four light-receiving areas. The light-receiving areas A to D of the outer peripheral photodetector 40 receive light of the outer peripheral portion of the reflected light beam 30 (hereinafter, referred to as “outer peripheral light”). The light-receiving areas A to D of the inner photodetector 41 receive light of the inner portion of the reflected light beam 30 (hereinafter, referred to as “inner light”). Additionally, outer peripheral light and inner light can be obtained by providing, for example, a beam splitter 47, a light shielding plate 48 for shielding inner light, and a light shielding plate 49 for shielding outer peripheral light of FIG. 3.
Each of the light-receiving areas generates photocurrent of a level corresponding to a quantity of received light and outputs photocurrent to the preamplifier 11. The preamplifier 11 has I/V converters 42a to 42d and 43a to 43d which correspond to the light-receiving areas, respectively, and convert received photocurrent to voltage. Converted voltage signals are outputted to an outer peripheral FE generator 44 and an inner FE generator 45.
The outer peripheral FE generator 44 generates an error signal based on the output signal of the preamplifier 11 according to the astigmatism method. The error signal indicates an error between the beam spot of outer peripheral light and the perpendicular direction of the optical disc 20. The error signal indicates a focus error of outer peripheral light and will be referred to as an “outer peripheral FE signal”. On the other hand, the inner FE generator 45 generates an error signal based on the output signal of the preamplifier 11 according to the astigmatism method. The error signal indicates an error between the beam spot of inner light and the perpendicular direction of the optical disc 20. The error signal indicates a focus error of inner light and will be referred to as an “inner FE signal”.
A focus error generator 7 sums the output signals of the outer peripheral FE generator 44 and the inner FE generator 45 and generates an error signal indicating an error between the perpendicular direction of the optical disc 20 and a beam spot generated by all the light beams outputted from the light source 3. The error signal is referred to as a so-called focus error signal and will be referred to an “FE signal” below. Although the generating method of the FE signal is somewhat different from that of the FE signal generated by the astigmatism method, the characteristics are the same.
The FE signal serving as an output signal of the focus error generator 7 is outputted to a focus actuator driver 9 after filter computing such as phase compensation and gain compensation is performed in a focus controller 17. The focus actuator driver 9 receives the signal processed by the focus controller 17 to generate a drive signal.
The objective lens 1 is driven by the focus actuator 2 which operates based on the drive signal from the focus actuator driver 9. A state of a beam spot is controlled so as to have a predetermined converging state on the information storage layer 209 of the optical disc 20, which realizes focus control.
Referring to FIGS. 16A and 16B, spherical aberration will be described below. FIG. 16A shows that no spherical aberration is produced at the information storage layer 29. FIG. 16B shows that spherical aberration is produced at the information storage layer 29.
In the state of FIG. 16A, a thickness DA from the surface of the optical disc 20 to the information storage layer is optimum relative to the light beam 30. In a state in which focus control is performed, a light beam emitted from the light source 3 is refracted on the protective layer 25 of the optical disc 20. The light beam on the outer periphery (outer peripheral light beam) 30-1 is collected on point C, and a light beam on the inner periphery (inner light beam) 30-2 is collected on point B. Position A is defined on a straight line passing through focus B and focus C and on the information storage layer 29. Since no spherical aberration is produced at the information storage layer 29 of the optical disc 20, the focus C of the outer peripheral light beam 30-1 and the focus B of the inner light beam 30-2 both coincide with the position A. Namely, an equidistant surface from the position A and the wave front of the light beam coincide with each other.
Meanwhile, in the state of FIG. 16B, a thickness (a thickness of the protective layer 25) DB from the surface of the disc to the information storage layer is smaller than the thickness DA of the protective layer 25. As a result, the focus C of the outer peripheral light beam 30-1 and the focus B of the inner light beam 30-2 are separated from each other, and the two focuses enter a defocusing state relative to the position A of the information storage layer 29 where the light beam 30 should be entirely converged. Namely, spherical aberration is produced. Then, as a thickness DB of the protective layer 25 decreases, the influence on the spherical aberration increases. In FIG. 16B, solid lines indicate the outer peripheral light beam 30-1 and the inner light beam 30-2 when spherical aberration is produced, and broken lines indicate the outer peripheral light beam and the inner light beam when no spherical aberration is produced.
However, even when spherical aberration is produced, focus control is performed so that an FE signal outputted from the focus error generator 7 becomes substantially 0. Thus, it can be said that the focal position A of the light beam 30 coincides with the information storage layer 29. Unlike FIG. 16A, the wave front of the light beam does not coincide with the equidistant surface from the position A.
The focus B and the focus C are separated from each other also when the protective layer 25 has a larger thickness than the thickness DA of the protective layer shown in FIG. 16A. The two focuses enter a defocusing state relative to the position A of the information storage layer 29. Thus, spherical aberration is produced.
Referring to FIG. 14 again, the following will describe spherical aberration control for correcting spherical aberration. The outer peripheral FE generator 44 and the inner FE generator 45 output an-outer peripheral FE signal and an inner FE signal including an influence quantity of spherical aberration on an outer peripheral light beam (a defocusing quantity of the focus C) and an influence quantity of spherical aberration on an inner light beam (a defocusing quantity of the focus B), respectively. A spherical aberration detector 31 calculates a difference between the outer peripheral FE signal and the inner FE signal by arithmetic and generates a signal corresponding to a quantity of spherical aberration produced at the convergence position A (hereinafter, referred to as a “spherical aberration signal”).
A spherical aberration controller 35 compensates the phase of the spherical aberration signal and performs filter computing such as gain compensation. Thereafter, the spherical aberration controller 35 outputs the processed spherical aberration signal to a beam expander driver 33. The beam expander driver 33 generates a drive signal based on the spherical aberration signal and impresses the drive signal to a correction actuator 34. The spherical aberration correction actuator 34 changes an interval between spherical aberration correction lenses 15 based on the drive signal and sets a spherical aberration substantially at 0. As a result, the focus C of the outer peripheral light beam and the focus B of the inner light beam coincide with the position A. In this manner, spherical aberration control is performed.
Referring to FIGS. 17A to 17E and FIGS. 18A to 18E, the following will describe the influence of the FE signal, which is obtained in the defocusing state, on the spherical aberration signal. In the following explanation, spherical aberration control is not performed.
FIG. 17A shows a cross sectional view of a light beam divided into the outer peripheral light beam 30-1 and the inner light beam 30-2 at the position of a 50% radius from the center of a received light beam. FIG. 17B shows the waveform of the outer peripheral FE signal. Similarly, FIG. 17C indicates the waveform of the inner FE signal, FIG. 17D shows the waveform of the FE signal, and FIG. 17E shows the waveform of the spherical aberration signal. In the graphs of FIGS. 17B to 17E, the vertical axes indicate the voltage levels of the signals, and the horizontal axes indicate defocusing quantities. As described above, regarding the outer peripheral FE signal of FIG. 17B and the inner FE signal of FIG. 17C, when addition is performed, the FE signal of FIG. 17D is obtained. When subtraction is performed, the spherical aberration signal of FIG. 17E is obtained.
When the outer peripheral light beam 30-1 and the inner light beam 30-2 are divided as shown in FIG. 17A, the outer periphery is larger in light quantity than the inner periphery. Thus, the outer peripheral FE signal of FIG. 17B is larger in amplitude than the inner FE signal of FIG. 17C. As a result, even though a spherical aberration remains constant, as shown in FIG. 17E, the level of spherical aberration signal changes according to a defocusing quantity. Besides, as is understood from FIGS. 17D and 17E, the polarity of the spherical aberration signal is the same as that of the FE signal. The spherical aberration signal is delayed by 0 degree from the phase of the FE signal.
Meanwhile, FIG. 18A indicates a cross sectional view of a light beam divided into the outer peripheral light beam 30-1 and the inner light beam 30-2 at the position of a 75% radius from the center of a received light beam. FIG. 18B indicates the waveform of the outer peripheral FE signal. Similarly, FIG. 18C indicates the waveform of the inner FE signal, FIG. 18D shows the waveform of the FE signal, and FIG. 18E shows the waveform of the spherical aberration signal. In FIGS. 18B to 18E, the vertical axes indicate voltage levels, and the horizontal axes indicate defocusing quantities.
When the outer peripheral light beam 30-1 and the inner light beam 30-2 are divided as shown in FIG. 18A, the inner periphery is larger in light quantity than the outer periphery. Thus, the inner FE signal of FIG. 17C is larger in amplitude than the outer peripheral FE signal of FIG. 17B. As a result, even though a spherical aberration remains constant, as shown in FIG. 18E, the level of spherical aberration signal changes according to a defocusing quantity. Besides, as is understood from FIGS. 18D and 18E, the spherical aberration signal is opposite in polarity to the FE signal. The spherical aberration signal is delayed by 180 degrees from the phase of the FE signal.
In the beam spot, a spherical aberration is produced according to a thickness of the protective layer 25 in proportion to the fourth power of the numerical aperture (hereinafter, referred to as “NA”) of the objective lens 1. In the case of a conventional optical disc (DVD, etc.) allowing an NA of about 0.6, a spherical aberration caused by an uneven thickness of the protective layer 25 is negligible within a permissible range.
However, for example, in the case of an optical disc such as a BD requiring the light source 3 having an NA of 0.85 and a wavelength of 405 nm, in order to obtain a high-quality information signal, the aberration of a light source, particularly a spherical aberration caused by the objective lens 1 and the protective layer 25 of the optical disc, may not be negligible.
Although a method using the spherical aberration signal to correct a spherical aberration has been considered, a spherical aberration caused by an uneven thickness of the protective layer 25 may not be accurately corrected, since the spherical aberration signal includes an error in the defocusing state.